Active sonar transmits a signal into the water, and receives echoes from targets in the water. The targets can include, but are not limited to, submarines, torpedoes, tethered mines, bottom mines, cables, and bottom features such as hills, trenches, and the bottom surface.
Active sonar systems have a variety of configurations. Bistatic active sonar systems have an acoustic transmitter separate from an acoustic receiver. Monostatic active sonar systems have an acoustic transmitter co-located with an acoustic receiver.
The acoustic receiver portion of an active sonar system can have an array of receive elements arranged in a receive array formed as a line or a curve. The receive elements may be regularly or irregularly spaced in the receive array. With this arrangement, signals provided by the receive elements can be added to provide a receive beam having a beamwidth inversely proportional to a length of the receive array. Relative time delays or relative phase shifts can be introduced to the signals provided by the receive elements to steer the receive beams about the receive array.
Where the receive array is a horizontal receive array, an azimuth width of a receive beam is inversely proportional to the length of the array in wavelengths. Therefore, high spatial resolution can be achieved either by lengthening the receive array or by increasing the frequency of operation. However, physical array length is often limited by a size of a platform to which the receive array is attached. Furthermore, high acoustic frequencies attenuate rapidly in the water, preventing acoustic propagation to long ranges. As a result, conventional active sonar systems are limited in performance by receive array length constraints and by acoustic frequency constraints.
Synthetic aperture sonar (SAS) has characteristics similar to synthetic aperture radar (SAR). SAS improves the spatial resolution of an active sonar array by combining data coherently between pings (acoustic pulses) to synthesize a longer effective array. For SAS processing, Nyquist sampling constraints require that the receive array advance (move along its axis) by no more than half the physical length of the receive array between successive pings. It is known that more rapid movement of the receive array results in formation of grating lobes. A variety of SAS algorithms are also known.
SAS requires an exceptionally good estimate of a spatial track (i.e., position or motion estimate) of the receive array with time, in order to be able to accurately add signals from the receive array coherently over the entire synthetic aperture. It will be understood that a spatial track is associated with six degrees of freedom of motion of the receive array: three displacements and three rotations. Several methods are known for estimation of the track or portions of the track of the receive array in SAS processing.
For example, a high-quality inertial measurement unit (IMU) can be used along with a Doppler velocity sonar and other ancillary equipment to provide track estimates. However, the IMU and the other ancillary equipment are costly, and require space and power that are at a premium for undersea receive arrays.
Furthermore, correlation (or differential phase) between receive signals provided by the individual receive elements of the receive array can be used to estimate differential motion between consecutive pings. This technique typically looks over a narrow azimuth and range sector, to reduce the dimensionality of the required motion estimate. This technique does not obtain precise track estimates for motion in all six degrees of freedom.
Still further, an auto-focus technique can further adjust the track estimate based on an ability of the adjustment to improve the quality of a resulting SAS image. The auto-focus method can compensate for an accumulation of errors in differential motion over the duration (i.e., time and length) of the synthetic aperture.